The invention relates to a method and an apparatus for diagnosing a coolant pump that can be switched independent of the operating condition of an internal combustion engine for circulating a coolant in a coolant circuit of the internal combustion engine.
Such methods and apparatuses are used to check the operation of a switchable coolant pump for faulty switching and to avoid damage to the coolant circuit and/or the internal combustion engine resulting therefrom. Switchable coolant pumps are used to implement an innovative thermal management (ITM) in cooling circuits of internal combustion engines, in particular to improve the heat-up of the engine following a cold start. Electrically drivable coolant pumps, coolant pumps that can be disconnected from a mechanical belt drive or potted coolant pumps may be used as switchable coolant pumps. With potted coolant pumps, a pot is placed over the impeller of the coolant pump, preferably controlled by a vacuum or hydraulically, allowing the coolant flow to be regulated.
DE 10 2008 032 130 A1 discloses a generic method for diagnosing a coolant pump that can be switched on and off regardless of the operating state of an internal combustion engine for circulating a coolant in a closed-loop coolant circuit of the internal combustion engine, wherein at a predetermined time after a detected cold start of the internal combustion engine, both a value representing a coolant temperature (TCO) of the combustion engine and a value representing the cylinder head temperature (CHT) of the internal combustion engine are determined, which are thereafter compared with each other. The functionality of the coolant pump is assessed depending on the result of the comparison. Disadvantageously, the compared values of coolant temperature and cylinder head temperature may be in a very wide load-dependent tolerance range, thus increasing the risk for misdiagnoses. For example, the temperature measured on the cylinder head increases disproportionately when the load is suddenly increased, whereas the coolant temperature at the outlet of the internal combustion engine adjusts only after a time delay. Such misdiagnoses can cause uncertainty for the operator of the internal combustion engine and unnecessary maintenance work on the coolant circuit.